The 3rd Generation Partnership Project (3GPP) specifies an EUTRAN (Evolved UMTS Terrestrial Radio Access Network. UMTS: Universal Mobile Telecommunications System) network referred to as Dual Connectivity, in which two eNode Bs (eNBs) and User Equipment (UE) transmit and receive packet data to and from one another.
FIG. 1 illustrates an example of a configuration of a wireless communication system achieving Dual Connectivity.
The wireless communication system illustrated in FIG. 1 includes UE 10, a Master eNode B (MeNodeB. Hereinafter, represented as a MeNB) 20, a Secondary eNode B (SeNodeB. Hereinafter, represented as a SeNB) 30, a Mobility Management Entity (MME) 40, and a Serving Gateway (S-GW) 50.
The MeNB 20 is a master cell base station.
The SeNB 30 is a small cell base station. Note that a cell under the control of the SeNB 30 (SCG: Secondary Cell Group) is located within a coverage area of a cell under the control of the MeNB 20 (MCG: Master Cell Group).
The UE 10 is a terminal that receives downlink (DL) packet data from the two of the MeNB 20 and the SeNB 30. Note that the UE 10 is to transmit uplink (UL) packet data either to only the MeNB 20, or to the two of the MeNB 20 and the SeNB 30.
The MME 40 is a core network apparatus that is arranged in a core network (CN), and performs transmission in a control (C-) plane and manages movement of the UE 10.
The S-GW 50 is a core network apparatus that is arranged in a CN, and transmits packet data in a user (U-) plane.
Note that the MeNB 20 is connected with the SeNB 30 via an X2 Interface, and the MME 40 and the S-GW 50 are connected with the MeNB 20 and the SeNB 30 via an S1 Interface.
FIG. 2 illustrates an example of a connection configuration of a C-plane in Dual Connectivity.
A C-plane connection is made as illustrated in FIG. 2. The UE 10 being in a connected state in Dual Connectivity has only a connection of S1-MME between the MeNB 20 and the MME 40. In addition, the UE 10 has only a Radio Resource Control (RRC) connection present in a wireless section between the UE 10 and the MeNB 20. In other words, no RRC connection is present in at least a wireless section between the UE 10 and the SeNB 30. However, the SeNB 30 may sometimes create signal information associated with a RRC message to the UE 10 and transmit the created signal information to the UE 10 via the MeNB 20.
In addition, examples of a connection configuration of a U-plane in Dual Connectivity include a configuration with Split bearer option and a configuration with SCG bearer option.
FIG. 3 illustrates an example of a connection configuration of a U-plane when configured with the Split bearer option. FIG. 4 illustrates an example of a connection configuration of a Radio Protocol when configured with the Split bearer option.
As illustrated in FIGS. 3 and 4, in the case of the configuration with the Split bearer option, U-plane DL packet data is transmitted from the S-GW 50 to only the MeNB 20, and is not transmitted to the SeNB 30. Note that, in the configurations illustrated in FIGS. 3 and 4, a bearer from the MeNB 20 to the UE 10 is referred to as an MCG bearer, and a bearer from the SeNB 30 to the UE 10 is referred to as a SCG bearer (the same applies to FIGS. 5 and 6 to be described later).
As illustrated in FIG. 4, the UE 10, the MeNB 20, and the SeNB 30 have a layer structure including a Packet Data Convergence Protocol (PDCP) layer, a Radio Link Control (RLC) layer, and a Medium Access Control (MAC) layer.
In the MeNB 20, U-plane DL packet data received from the S-GW 50 is accepted at a PDCP layer. Herein, one side of the PDCP layer of the MeNB 20 (right side of the layer in FIG. 4) can transmit a certain part of packet data (PDCP Protocol Data Unit (PDU)) to the UE 10 via a cell under the control of the MeNB 20, and can transmit a certain part of packet data (PDCP PDU) to the UE 10 via the SeNB 30. In other words, the PDCP layer of the MeNB 20 can split U-plane packet data.
In such a configuration with the Split bearer option, flow control is introduced, whereby the MeNB 20 feeds back a flow control signal from the SeNB 30 and adjusts, by using the flow control signal, an amount of DL packet data (an amount of PDCP PDUs) to be transmitted to the SeNB 30 for a purpose of sufficiently utilizing resources of the SeNB 30 without oppressing the resources of the SeNB 30.
The flow control signal includes information that indicates a status of the SeNB 30 in transmission of DL packet data received from the MeNB 20 to the UE 10, and information that indicates an amount of remaining buffer of the SeNB 30. Besides, the flow control signal may include, for example, information relating to transmission power of the SeNB 30, a number of bearers that the SeNB 30 can accommodate, and a maximum bit rate that the SeNB 30 can accommodate.
Herein, a mechanism of flow control is described below using a specific example.
The MeNB 20 transmits packet data with PDCP sequence number (SN) #100, #102, #104, #106, and #108 to the UE 10 via a cell under the control of the MeNB 20. On the other hand, the MeNB 20 transmits packet data with PDCP SN #101, #103, #105, #107, #109, and #111 to the SeNB 30.
Assume that the SeNB 30 has received all packet data with PDCP SN #101, #103, #105, #107, #109, and #111 and has successfully transmitted all the packet data to the UE 10. In addition, assume that the SeNB 30 has determined that all the packet data have been transmitted by receiving RLC Acks from the UE 10. In this case, the SeNB 30 feeds back, to the MeNB 20, an amount of remaining buffer of the SeNB 30 together with SN#111 as a PDCP SN for which a RLC Ack has been received last in order from the UE 10, as a flow control signal. Herein, by showing SN #111 as a “PDCP SN for which a RLC Ack has been received last in order from the UE 10” to the MeNB 20, the MeNB 20 can determine that all the packet data with PDCP SN #101, #103, #105, #107, and #109 have been successfully transmitted to the UE 10. Note that reception of a RLC Ack from the UE 10 is equivalent to reception of a Status PDU (or Status Report) that is called in NPL 1 (3GPP TS 36.322 V12.0.0).
Upon determining that all of the packet data (PDCP PDUs) transmitted to the SeNB 30 have been transmitted to the UE 10, the MeNB 20 looks at the amount of remaining buffer of the SeNB 30 and adjusts an amount of packet data (an amount of PDCP PDUs) to be next transmitted to the SeNB 30.
Note that the configuration with the SCG bearer option out of the above-described U-plane connection configurations in Dual Connectivity is irrelevant to the present invention, but is briefly described below for reference.
FIG. 5 illustrates an example of a connection configuration of a U-plane when configured with the SCG bearer option. FIG. 6 illustrates an example of a connection configuration of a Radio Protocol when configured with the SCG bearer option.
As illustrated in FIGS. 5 and 6, in the case of the configuration with the SCG bearer option, U-plane DL packet data is transmitted to both the MeNB 20 and the SeNB 30 from the S-GW 50, and is transmitted to the UE 10 via cells respectively under the control of the MeNB 20 and the SeNB 30.
In such a configuration with the SCG bearer option, packet data transmitted and received between the CN and the UE 10 never passes through X2-U. However, in a case of, for example, addition and deletion of the SeNB 30, X2-U is used in order to carry out data forwarding for forwarding packet data remaining in one of the MeNB 20 and the SeNB 30 to the other.